This study provides detailed information on the effects of particle size, respiratory parameters, and constitutional variables on particle deposition efficiency at the various deposition sites in the human respiratory tract. Monodisperse ferric oxide microspheres tagged with gamma-emitting isotopes are deposited in hollow bronchial airway casts under simulated breathing conditions, and deposition efficiency is measured at the various branching levels using collimated scintillation detectors, and by optical and scanning electron microscopy. The patterns and efficiency of deposition at various branching levels are related to the flow profiles and turbulence intensities within the cast airways as measured by hot-wire anemometry. The data are used to test predictive mathematical models for lung deposition which have been proposed, and to generate improved models as they are needed.